Large and small plants of creosotebush and mesquite were subjected to drought during summer 1993. Large and small plants responded to drought with lowered xylem water potential and lowered photosynthetic gas exchange. Large plants appear to maintain a reduced but constant photosynthetic rate during drought, and responded to precipitation with increased photosynthesis. How-ever, small plants attempted to maintain a high photosynthetic rate throughout the season, responding less strongly to  precipitation events, but probably depleting reserves more and causing photosynthesis to decline more when droughted. Our results suggest that  large, established plants are better able to withstand drought than are small, establishing shrubs.